Prior art solar electric power supplies for use on space satellites today use flat plate photovoltaic modules in which large area silicon or gallium arsenide (GaAs) solar cells are assembled like tiles side-by-side to form large area single crystal arrays. These power supplies are made lighter by making the single crystal cells thinner and thinner with the consequence that they become more and more fragile and expensive. The area related power density for these power supplies is limited by the cell energy conversion efficiencies which are generally less than 20%. Since GaAs flat plate solar cells are more efficient and more radiation resistant than silicon flat plate cells, GaAs electric power supplies are preferred to silicon for many applications. However, GaAs cells are heavier and much more expensive.
In order to overcome many of the limitations in today's satellite solar electric power supplies, O'Neill (NASA SBIR--1986) proposed a power module consisting of an array of mini-domed Fresnel lenses which concentrate sunlight onto small area standard thickness GaAs concentrator cells. Each mini-domed lens concentrates the sunlight to a point-focus at which point a small GaAs cell is located. The lenses are made very thin and lightweight and since the GaAs cells are very small, they are very durable, lightweight, and less expensive. Given the concentrated sunlight concept, Fraas and Avery (1990) then proposed a tandem concentrator solar cell consisting of a GaAs light sensitive cell stacked on top of an infrared sensitive GaSb solar cell. The demonstrated energy conversion efficiency of the resultant cell-stack is 30%. These two concepts have now been combined (Pisxcxor et. al.--1990, Fraas et. al.--1992) to produce a point-focus concentrator module incorporating high efficiency tandem-cells. The resultant power supply (when compared with a silicon flat plate power supply producing the same amount of electric power) is one-third the size and half the weight of the silicon power supply.
While the performance of the above point-focus mini-domed lens module is quite impressive, there are still several disadvantages inherent in this design. First, the point-focus lens in its present form is not durable in the space environment. Second, the present mini-dome lens is very expensive to manufacture. Finally, third, the point-focus module must be pointed at the sun with two-axis tracking. In the following paragraphs, these problems are described in more detail.
The optical design of the point-focus mini-domed Fresnel lens requires the Fresnel prisms on the inner surface of the lens to be reentrant such that they are interlocking with the molding tool. As a result of this fact, the lens is molded from transparent silicone rubber. After the rubber lens is cured, it is removed from the mold by stretching the lens. However in space, the silicone rubber is attacked by ultraviolet (UV) rays, and in low earth orbit, by atomic oxygen. Hence, the rubber lens must be protected in space. Two solutions to this degradation problem have been proposed. The first solution involves adhesive bonding the rubber lens inside a domed glass superstrate and the second solution involves coating the lens with a protective multilayer coating. The forming, trimming and lamination of the glass superstrate to the silicone lens has proved to be a critical manufacturing problem. Costs are high and reproducibility is poor. Coated lenses have been fabricated but the coating crazes in handling and thermal cycling leading to poor optical transmission. These point-focus lens problems suggest that the point-focus module will not be durable and will be very expensive to manufacture. The line-focus module described herein bypasses these problems.
While the point-focus lens array with tandem cell-stacks offers the highest efficiency of any present space solar array technology, there are many missions which are not best served by the mini-dome lens array approach. For example, some missions can provide only single-axis sun-tracking for the solar array. The mini-dome point-focus lens array requires two-axis sun-tracking, and will not work for such missions. However, line-focus concentrators will work with single-axis sun-tracking.
Therefore, it is highly desirable to produce a line-focus electric power supply incorporating high performance tandem cell-stacks so that it can serve a larger number of missions including those where only single-axis sun-tracking is available. Furthermore, it is necessary to produce low cost lenses which are easy to manufacture and durable in space against UV and atomic oxygen. Finally, for radiation orbits, it is also desirable to protect the cells against radiation damage. The line-focus photovoltaic module incorporating tandem cell-stacks described herein has these beneficial characteristics.